Solid state cell with anolyte

ABSTRACT

A solid state cell having a solid cathode, a solid electrolyte, and a solid anolyte comprised of at least 50% by volume of ionically conductive materials such as the electrolyte and 50% or less by volume of an active metal. The anolyte is either the cell anode or alternatively the anolyte is an additional structural member within said cell positioned between an anode, comprised of the same active metal, and the solid electrolyte.

This is a division of application Ser. No. 262890, filed 5-13-81 nowU.S. Pat. No. 4,390,446.

This invention relates to solid state cells having active metal anodessuch as lithium and particularly those cells having anodes with includedelectrolyte.

Solid state cells in which all of the components including theelectrolyte are solid during cell operation have certain advantages anddisadvantages. The solid state cells are immune to detrimentalelectrolyte leakage which tends to plague fluid electrolyte cells.Additionally the solid state cells have extremely long shelf livesgenerally in excess of ten years. Furthermore, the solid state cells,even with active metal anodes, are extremely stable even under abusiveconditions when compared to their fluid electrolyte cell counterparts.As a result, solid state cells are ideal for use with for exampledelicate electronic componentry and human implanted heart pacemakers.However, the very nature of the solid electrolyte which provides theadvantageous stability constrains such cells to relatively low dischargecapabilities when compared to fluid electrolyte cells. Solid ionicconduction is very slow when compared to ionic conduction in fluids withthe practical cell discharge rate being determined by the ionicconductivity of the electrolyte. The electrolytes used in high capacityand high voltage active metal anode solid state cells (because ofchemical compatibility with the anode) have particularly low ionicconductivities and practical discharge capabilities (in the lowmicroampere or microwatt region).

The construction of a solid state cell generally requires highcompacting pressure of components or other similar means to provideintimate electrode-electrolyte interfaces whereby the solid ionconduction is possible. At relatively high drain rates solid state celllife is drastically shortened as a result of disruption of theinterface, with mechanical and electrical disconnection between theelectrolyte and the electrodes, particularly the anode. During dischargeof a solid state cell having, for example, a lithium anode, lithiumcations at the anode-electrolyte interface leave the interface, travelthrough the solid electrolyte to the cathode at which site the cellreaction takes place. The departing lithium cations leave vacanciesbehind which are continuously filled by additional diffusing lithiumfrom the bulk of the anode. At higher drain rates, however, thedeparture of the lithium cations causes the vacancies to be formedfaster than they can be filled with resultant coalescing of thevacancies to form irreparable, unbridgeable gaps at theanode-electrolyte interface. These gaps further increase localizedinterfacial vacancies by shunting cation flow through the remainingreduced interfacial contact area. As a result, gap formation rapidlyincreases with a mechanical and electrical disconnection or delaminationbetween anode and electrolyte occurring which severely curtails cellcapacity. Active anode metals such as lithium which have relatively poorself diffusion rates are particularly constrained to low discharge ratesup to the limiting discharge rate at which vacancy formation (withcoalescing into detrimental gaps) occurs at a rate faster thanadditional cation filling of the vacancies.

It is an object of the present invention to increase solid state cellcapacity at relatively high discharge rates. This and other objects,features and advantages of the present invention will become moreevident from the following discussion as well as the drawings in which:

FIG. 1 is a graphic representation of discharge curves of cellsembodying the present invention;

FIG. 2 is a graphic representation of discharge curves of cells of asecond embodiment of the present invention;

FIG. 3 is a discharge curve comparison between batteries comprised ofthe cells depicted in FIGS. 1 and 2; and

FIG. 4 is a discharge curve comparison between a cell of the prior artand a cell as depicted in FIG. 2.

Generally the present invention comprises a solid state cell having asolid cathode, a solid electrolyte and an active metal solid anolyte.The solid anolyte is comprised of at least 50% by volume of an ionicconductor such as the electrolyte and 50% or less by volume of theactive metal. The anolyte either constitutes the entire active anode forthe cell or is present as an additional structural member interposedbetween the active metal anode and the solid electrolyte. It has beendiscovered that the massive inclusions of electrolyte (50% or more byvolume) with the solid anolyte, particularly when the anolyteconstitutes the entire anode, dramatically increases dischargeable cellcapacity at relatively high discharge rates (e.g. above about 0.25 mW)despite the severe reduction of active metal content within the anolyte(an anolyte comprised of 50% by volume lithium metal and 50% by volumeof a LiI, LiOH, Al₂ O₃ (LLA) electrolyte is only about 13% by weightlithium) as compared to prior art cells having pure active metal anodes.It has in fact been further discovered that additional increases inelectrolyte content with concomitant reduction in active metal contentstill further increases the high rate capacity. Thus a cell having 33%by volume lithium anolyte (about 7% by weight) with LLA electrolyteprovides more than double the capacity of cells having pure lithiumanodes at the higher drain rates.

Though not as effective at high drain rates as the anolyte whichconstitutes the entire anode, a thin layer solid anolyte positionedbetween the active metal anode and the solid electrolyte has theadvantage of retaining substantially all of the low drain rate capacityif the layer is very thin while enhancing high rate drain capacity.Because of its smaller dimensions it is preferred that the electrolytecontent in the interposed thin anolyte layer be greater than the amountutilized in the anolyte which constitutes the entire active anode. Thepercentage of the electrolyte to be used in the anolyte layer isgenerally determined by the thickness of the layer and ranges from about95% by volume for very thin layers (1 mil) to the 50% such as when theanolyte layer comprises the entire anode.

It is postulated that the solid anolyte increases interfacial contactbetween the active anode metal (whether within the anolyte or adjacentthereto) and the electrolyte thereby reducing the likelihood ofcoalescing of vacancies into the detrimental and irreparable gaps. Theanolyte which also comprises the anode further enhances cation travelrates of the anode metal since the ionic conductivity of the electrolyteis greater than the self diffusion rate in active metals such aslithium. Because of greater surface area contact the anolyte furtherprovides shorter diffusion paths, and as a result the vacancies arefilled at a greater rate before they can form the irreparable gaps.Finally the anolyte decreases the sharp interface between anode andelectrolyte present in prior art cells whereby the occurrence ofmechanical and electrical disconnection or delamination between anodeand electrolyte is significantly reduced or delayed. In order to providesuch effect it is essential that the electrolyte (or ionic conductor)content within the anolyte be massive with a 50% by volume inclusionconstituting a lower limit for the electrolyte content. Thus variousprior art patents such as U.S. Pat. Nos. 3,455,742; 3,730,775; and3,824,130 which peripherically suggest the inclusion of electrolyte inthe anode do not however suggest the massive inclusions of the presentinvention wherein the active metal of the anode actually becomes a minorpart thereof and wherein unexpectedly markedly superior performancecharacteristics are achieved thereby. Additionally, the patents do noteven hint at the utilization of an additional structural element of athin anolyte layer as in one embodiment of the present invention.

The active metals used in the solid anolytes of the present invention asthe anode active material are generally metals above hydrogen in the EMFseries and more specifically alkali metals particularly lithium. Thematerials used as the electrolyte of the cell and as the conductiveanolyte component need not be the same but both should have electrolyteconductivity characteristics; i.e., ionic conductivity greater than theequivalent self diffusion rate of the active anode metal, generallygreater than 1×10⁻⁷ ohm⁻¹ cm⁻¹ at room temperatures, and should bechemically compatible with the anode metal. The material utilized in theanolyte may however be electronically conductive which property wouldrender it unfit (since it would cause an internal cell short circuit)for use as the cell electrolyte itself. It is, however, preferred thatthe cell electrolyte itself be the material which is admixed with theactive metal in the anolyte. The most preferred electrolyte material foruse in active metal solid state cells (and in the anolyte and as theelectrolyte of the present invention) is LiI particularly when admixedwith Al₂ O₃ and/or LiOH (LLA as described above) or with other suitabledopants which increase ionic conductivity. Other highly ionicallyconductive materials such as Na_(x) WO₃ (x<1) are utilizable as theconductive materials in the anolyte, particularly with a sodium activeanode metal. However, Na_(x) WO₃ is also electronically conductivewhereby its use as the electrolyte for the cell is precluded.

Solid active cathode materials utilized in the cells of the presentinvention generally comprise those materials commonly used in solidstate cells such as metal halides; metal chalcogenides; halogens i.e.iodine; metal oxides; chalcogens i.e. sulfur, selenium and tellurium;and mixtures such as the PbI₂ /PbS cathode described in U.S. Pat. No.3,959,012 and the TiS₂ /S cathode described in copending applicationSer. No. 129,144 now U.S. Pat. No. 4,263,377. Because of theirparticularly high energy densities, cathodes as described in theaforementioned patent and application are preferred as cathodes forcells having the anolytes of the present invention.

In order to more clearly illustrate the efficacy of the presentinvention the following examples are presented. Details contained withinsuch examples are however, not to be considered as limitations on thepresent invention. All of the cells in the following Examples 1-5 havecathodes which are 73.9% TiS₂, 21.1% S and 5% LiI by weight and are ofsubstantially the same dimensions i.e. 1.25" (3.18 cm) diameter by0.040" (0.10 cm) height. Because of the volumetric constraints imposedby specific cell sizes, comparisons wherever made between cells are madeon a volumetric rather than a gravimetric basis. Weights of specificcomponent materials are provided for complete illustrative purposes.Unless otherwise indicated all parts are parts by weight.

EXAMPLE 1

Three cells each comprised of a solid cathode weighing 0.650 gm with asurface area of 7.92 cm² ; a solid anolyte of 50% by volume lithiummetal and 50% by volume LLA weighing 0.650 gm (13.2% Li and 86.8% LLA)with a surface area of 7.42 cm² ; and a solid LLA electrolytetherebetween are discharged at room temperature. The cells aredischarged at the different rates of 300, 250 and 200 μwatts asindicated in FIG. 1 and provide discharge times of about 400, 800 and1100 hours respectively.

EXAMPLE 2

Four cells each comprised of a solid cathode weighing 0.900 gm with asurface area of 7.92 cm² ; a solid anolyte of 33% by volume lithiummetal and 67% by volume LLA weighing 0.530 gm (7.04% Li and 92.96% LLA)with a surface area of 7.42 cm² ; and a solid LLA electrolytetherebetween are discharged at room temperature. The cells aredischarged at the different rates of 250, 225, 200 and 175 μwatts asindicated in FIG. 2 and provide discharge times of about 800, 1175, 1400and 1900 hours respectively.

EXAMPLE 3

Two batteries, with each having two cells in parallel are made. Thecells of one battery are each comprised of a 700 mg cathode, a 650 mgsolid anolyte of 50% by volume LLA and 50% by volume Li and a LLAelectrolyte. The cells of the other battery are each comprised of a 700mg cathode, a 800 mg anolyte of 33% by volume Li and 67% by volume LLAand a LLA electrolyte. The batteries are discharged at 600 μwatts atroom temperature with the results shown in FIG. 3. The cell having the33% by volume lithium (56.32 mg) anolyte provides a discharge time ofabout 675 hours and the cell having the anolyte with 50% by volumelithium (85.8 mg) providing about 465 hours of discharge time. It isaccordingly evident that at the higher rates, cells having the lesservolume of lithium, with concomitant greater amounts of electrolyteinclusion, are markedly superior.

EXAMPLE 4

Two cells with each having a cathode weighing 0.90 gm and an LLA solidelectrolyte but with one cell having a lithium foil anode weighing 0.113gm in accordance with prior art and the other having an anode weighing0.530 gm comprised of 33% by volume lithium (0.0373 gm) and 67% byvolume LLA electrolyte. The cells are discharged at room temperature ata discharge rate of 250 μwatts with the comparative discharge curveshown in FIG. 4. With about one-third the lithium by weight in the anodethereof the cell with the 33% by volume lithium anode provides more thandouble the discharge capacity of the prior art cell.

EXAMPLE 5

Nine batteries are made with each having the dimensions 1.3" (3.3 cm)O.D.×0.1" (0.25 cm) Ht and comprised of 2 cells in parallel. The cellsare each comprised of 0.90 gm cathodes and LLA electrolyte. The cells ofthree of the batteries (1-3) contain 1.113 gm lithium foil anodes inaccordance with the prior art. The cells of three batteries (4-6)contain 0.530 gm, 33% by volume Li and 67% by volume LLA, anodes. Thecells of the last three batteries (7-9) contain 0.650, gm 50% by volumeLi and 50% by volume LLA, anodes. All the batteries are discharged atvarying power drains with the comparative realizable energy densities atthe indicated power drains shown in the following table:

Realizable Energy Density (W.hrs/in³) at Indicated Power Drain

    ______________________________________                                        Batteries                                                                            Power Drain:                                                                              0.05 mW   0.3 mW  0.5 mW                                   ______________________________________                                        1-3    Prior Art   7.5       0.5     0.1                                      4-6    33 v/o Li anode                                                                           6.8       6.0     3.3                                      7-9    50 v/o Li anode                                                                           9.8       5.7     2.6                                      ______________________________________                                    

EXAMPLE 6 (PRIOR ART)

A cell is constructed with a lithium anode (0.04 gm foil) an LLAelectrolyte and a 0.100 gm cathode comprised of 80% TiS₂ and 20% S. Theanode and cathode geometric areas are 1.48 cm² and 1.77 cm²respectively. The cell is discharged at room temperature under aconstant load of 23 kohm (about 175 microwatts). The cell shows anabrupt decrease in load voltage after about 20 hours.

EXAMPLE 7

A cell is made in accordance with Example 6 but with the addition of a50 mg layer (about 5 mils) of a mixture of LLA and 4% lithium (about 20%by volume) interposed between anode and electrolyte. The cell isdischarged as in Example 6 and shows no abrupt decrease in load voltageeven after 300 hours of discharge (about 25 mAh). The dischargedcapacity is well in excess of the additional lithium capacity in thelayer added to the capacity of the prior art cell of Example 6.

It is evident from the above Examples that the cells of the presentinvention having solid anolytes therein are substantially better interms of high rate performance than cells of the prior art. It isfurther evident that at the increasingly higher rates, cells havinglesser amounts of anode metal in the anolyte are clearly superior incapacity. It is however understood that the above Examples werepresented for illustrative purposes only. Accordingly, changes may bemade in cell components, structure, relative proportions of materialsand the like without departing from the scope of the present inventionas defined in the following claims.

What is claimed is:
 1. A solid state cell comprising a solid cathode, asolid electrolyte and a solid anolyte characterized in that said anolyteis comprised of a solid active metal anode material admixed with acationic conductor and wherein said cationic conductor comprises atleast 50% by volume of said anolyte.
 2. The solid state cell of claim 1wherein said solid anolyte comprises a layer interposed between andintimately contacted with said solid electrolyte and an active metalanode.
 3. The solid state cell of claim 1 wherein said solid anolytecomprises the anode for said cell.
 4. The solid state cell of claim 2wherein said cationic conductor is the material which comprises saidelectrolyte.
 5. The solid state cell of claim 3 wherein said cationicconductor is the material which comprises said electrolyte.
 6. The solidstate cell of claim 2 wherein said cationic conductor comprises up to95% by volume of said anolyte.
 7. The solid state cell of claim 3wherein said cationic conductor comprises up to 67% by volume of saidanolyte.
 8. The solid state cell of claims 1 or 2 or 3 wherein saidactive metal of said anode and said anolyte is lithium.
 9. A solid statecell comprising a solid cathode comprised of a member of the groupconsisting of metal chalcogenides, metal halides, chalcogens, halogens,metal oxides and mixtures thereof; a solid electrolyte comprised of LiI;and a solid anolyte comprised of lithium and said solid electrolytewherein said anolyte comprises the anode for said cell and wherein saidsolid electrolyte in said anolyte is at least 50% by volume thereof. 10.The solid state cell of claim 9 wherein said solid electrolyte in saidanolyte comprises 67% by volume thereof.
 11. The solid state cell ofclaim 9 or 10 wherein said cathode is comprised of an admixture of TiS₂and S.